Section 6 Ion Implantation Jaeger Chapter 5 EE143 Ali Javey Ion Implantation Overview Wafer is Target in High Energy Accelerator Impurities Shot into Wafer Preferred Method of Adding Impurities to Wafers Wide Range of Impurity Species Almost Anything Tight Dose Control A few vs 20 30 for high temperature pre deposition processes Low Temperature Process Expensive Systems Vacuum System EE143 Ali Javey Force on charged particle Equipment Magnetic Field B 2mV qr 2 F q v x B T Implanted Dose EE143 Ali Javey 1 Q I t dt mqA 0 Ion Implantation C x y x as implant depth profile Blocking mask Si Equal Concentration contours Depth x Reminder Reminder During Duringimplantation implantation temperature temperatureisisambient ambient However However o post implant post implantannealing annealingstep step 900 900oC C isisrequired requiredto toanneal annealout outdefects defects EE143 Ali Javey Advantages of Ion Implantation Precise control of dose and depth profile Low temp process can use photoresist as mask Wide selection of masking materials e g photoresist oxide poly Si metal Less sensitive to surface cleaning procedures Excellent lateral uniformity 1 variation across 12 wafer Application example self aligned MOSFET source drain regions As As As Poly Si Gate n p Si EE143 Ali Javey n SiO2 Ion Implantation Energy Loss Mechanisms Nuclear stopping Si Si Crystalline Si substrate damaged by collision e Electronic stopping Si e Electronic excitation creates heat EE143 Ali Javey Ion Energy Loss Characteristics Light ions at higher energy more electronic stopping Heavier ions at lower energy more nuclear stopping EXAMPLES Implanting into Si H Electronic stopping dominates B Electronic stopping dominates As Nuclear stopping dominates EE143 Ali Javey Stopping Mechanisms EE143 Ali Javey Electronic Nuclear Stopping Damage Sn dE dx n Se dE dx e Depth x E 0 Se Surface E Eo Substrate A Se Sn Sn More damage at end of range Sn Se Less damage Se Sn x Rp EE143 Ali Javey Eo incident kinetic energy Simulation of 50keV Boron implanted into Si EE143 Ali Javey Model for blanket implantation Gaussian Profile x R p 2 N x N p exp 2 2 R p R p Projected Range R p Straggle Dose Q N x dx 2 N p R p 0 EE143 Ali Javey Projected Range and Straggle Rp and Rp values are given in tables or charts e g see pp 113 of Jaeger Note this means 0 02 m EE143 Ali Javey Selective Implantation N x y N x F y y a y a 1 F y erfc erfc 2 2 R 2 R R transverse straggle N x is one dimensional solution EE143 Ali Javey Transverse or Lateral Straggle Rt or R Rt Rp 1 Rt Rp Rt EE143 Ali Javey Feature Enlargement due to lateral straggle y Mask x x Rp Lower concentration Higher concentration Implanted Implantedspecies species has haslateral lateraldistribution distribution larger largerthan thanmask maskopening opening C y at x Rp y EE143 Ali Javey Definitions of Profile Parameters 1 Dose 0 C x dx 1 x C x dx 2 Projected Range R p 0 1 2 3 Longitudinal Straggle R p x R p 2 C x dx 0 4 Skewness M x R 3C x dx M 3 0 or 0 p 0 describes asymmetry between left side and right side 5 Kurtosis x R C x dx 0 4 p C x Kurtosis characterizes the contributions of the tail regions EE143 Ali Javey Rp x Selective Implantation Mask thickness Desire Implanted Impurity Level to be Much Less Than Wafer Doping N X0 NB or N X0 NB 10 EE143 Ali Javey Transmission Factor of Implantation Mask Mask material e g photoresist C x What fraction of dose gets into Si substrate Si substrate x 0 C x x d Mask material with d x 0 EE143 Ali Javey x d Transmitted Fraction d C 0 T 0 C x dx x dx Rp Rp d Rp 1 erfc 2 2 Rp 2 x y2 erfc x 1 e dy 0 are values of for ions into the masking material Rule of thumb Good masking thickness d Rp 4 3 Rp C x d 4 10 C x Rp EE143 Ali Javey Junction Depth The junction depth is calculated from the point at which the implant profile concentration bulk concentration N x j N B x j R p 2 N p exp N B 2 2 R p x j R p R p EE143 Ali Javey Np 2 ln NB Sheet Resistance RS of Implanted Layers Example n n type dopants implanted into p type substrate RS x 0 x xj p sub CB x 1 q x C x CB dx xj C x log scale 0 n CB p Total doping conc EE143 Ali Javey xj x Approximate Value for RS If C x CB for most depth x of interest and use approximation x constant 1 1 This expression assumes ALL Rs xj implanted dopants are 100 q q 0 C x dx electrically activated 1 Rs q use the for the highest doping region which carries R ohm s most of the current or ohm square EE143 Ali Javey Example Calculations 200 keV Phosphorus is implanted into a p Si C B 1016 cm3 with a dose of 1013 cm2 From graphs or tables Rp 0 254 m Rp 0 0775 m a Find peak concentration Cp 0 4 x 1013 0 0775 x10 4 5 2 x1017 cm3 b Find junction depths b Cp exp xj 0 254 2 2 Rp2 N CBB with xj in m R p xj 0 254 2 2 0 0775 2 ln 5 2 1017 1016 or xj 0 254 0 22 m xj1 0 032 m and xj2 0 474 m x x j1 Phosphorus Implant j2 p substrate 1E16 cm3 c Find sheet resistance From the mobility curve for electrons using peak conc as impurity conc n 350 cm2 V sec 1 1 1780 square Rs q n 1 6 10 19 350 1013 EE143 Ali Javey Channeling EE143 Ali Javey Use of tilt to reduce channeling Random component C x channeled component Lucky ions fall into channel despite tilt x Random Planar Channeling Axial Channeling o To we wafer Ali Javeyby Tominimize minimizechanneling channeling EE143 wetilt tilt wafer by77owith withrespect respectto toion ionbeam beam Prevention of Channeling by Pre amorphization Step 1 Si High dose Si implantation to covert surface layer into 2 1 E15 cm amorphous Si Step 2 Implantation of desired dopant into amorphous surface layer Si crystal Amorphous Si Si crystal B Disadvantage Disadvantage Needs Needsan anadditional additionalhigh dose high doseimplantation implantationstep step EE143 Ali Javey Kinetic Energy of Multiply Charged Ions With Accelerating Voltage x kV Singly charged Doubly charged Triply charged B P As Kinetic Energy x keV B Kinetic Energy 2x keV B Kinetic Energy 3x keV Note Kinetic energy is expressed in eV An electronic charge q experiencing a voltage drop of 1 Volt will gain a kinetic energy of 1 eV EE143 Ali Javey Molecular Ion Implantation Kinetic Energy x keV B F F BF2 accelerating voltage x kV B has 11 amu F has 19 amu Molecular ion …
View Full Document
Unlocking...